Dialkyl carbonates are important intermediates for the synthesis of fine chemicals, pharmaceuticals and plastics and are useful as synthetic lubricants, solvents, plasticizers and monomers for organic glass and various polymers, including polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transparency, shock resistance and processability.
Industrially, dimethyl carbonate (DMC) is used in the production of polycarbonates and has the potential to be used as an environmentally friendly fluid for numerous solvent-related applications and conceivably even as a fuel oxygenate (e.g., methyl tertiary butyl ether replacement).
Historically, DMC was prepared from the highly toxic intermediate phosgene, COCl2. Currently, it is prepared via oxidative carbonylation of methanol using a copper(I) chloride catalyst together with a halogen mitigation step using HCl. This method is based on copper(I) chloride as the catalyst and demonstrated in EP 534,545 B1 and EP 460,732 A1. The overall copper catalyzed reaction is shown in equation (1) below:CO+2CH3OH+½O2→(CH3O)2CO+H2O   (1)
The copper(I) chloride catalyst is very insoluble in this system and, thus, is a limiting component in the catalytic cycle. Hydrochloric acid is also added as a component in this oxidative carbonylation system during a mitigation step. This was done to prevent the oxidation of Cu(I) to Cu(II) in the presence of oxygen and water since Cu(I) is believed to be the active species in this system. This copper chloride catalyst-based oxidative carbonylation system, which is run between 120° C. and 160° C., is extremely corrosive and requires costly components (e.g., glass lined reactors). Failure in the glass lining could lead to rupture or explosion. Two other notable processes for the production of DMC are disclosed in U.S. Pat. Nos. 6,010,976 and 5,498,744. U.S. Pat. No. 6,010,976 discloses a catalytic reaction of urea with methanol to first form the carbamate, which is further reacted to form DMC, ammonia and carbon dioxide. U.S. Pat. No. 5,498,744 discloses a process that reacts methylnitrite with carbon monoxide over a catalyst to form DMC and (NO)x which is toxic.
DMC, due to its low toxicity and low atmospheric reactivity, has tremendous growth potential as a possible replacement for methyl tertiary butyl ether (MTBE), as a fluorocarbon solvent replacement in the electronics industry and as an environmentally friendly solvent for use in the production of polycarbonates. The problems with MTBE and fluorocarbons, and phosgene are widely publicized. The growth of DMC use has been, in part, limited by the difficulties in commercial production. An efficient and environmentally friendly method for the large-scale production of DMC would be highly desirable, especially a process that eliminates the need for a chloride-based catalyst and hydrochloric acid mitigation, which causes corrosion of the reaction vessel and impurities in the resultant product.
Accordingly, Applicants have developed an improved low corrosive process for the production of alkyl carbonates, and, in particular, DMC, starting from carbon monoxide, oxygen and alcohol in the presence of a triesterification catalyst, wherein the halogen (e.g., chlorine) concentration of the alkyl carbonate product is 5 ppm or less.